The temperature at which a bitumen is mixed with aggregate to produce an asphalt composition ready for use on roads is normally in the range of 140 to 170.degree. C., although some documents, e.g. U.S. Pat. No. 3,832,200, teach that even higher temperatures should be applied. Since, it is nowadays well appreciated that such hot bitumens may create potential health, safety and environmental hazards, much effort is directed in this field of technology to the development of asphalt compositions which can be handled at lower temperatures.
In this respect reference can be made to the application of bitumen emulsions which are prepared by mixing a hot bitumen with an aqueous emulsifier solution. These bitumen emulsions can normally be mixed with aggregate at a temperature much lower than 140.degree. C., in which way the above-mentioned hazards are much better controlled. Asphalt compositions with relatively high void contents, so-called open-graded asphalt, have the advantage that they are highly permeable to water. This means that water can run away more quickly from the road surface during service. A disadvantage of the high void content is that open-graded asphalt loses coarse aggregate from the road surface rather easily, so called fretting of the road surface. It further tends to lose internal cohesion more easily which will eventually lead to collapse of the material and lack of internal stability, often visible as deformation of the road surface, i.e. rutting. Moreover, strength of these asphalts is only developed slowly.
U.S. Pat. No. 5,114,483, U.S. Pat. No. 4,762,565 and EP-A-589,740 relate to the preparation of asphalt with the help of an emulsion of a soft binder component and an emulsion of a hard binder component. The process of the present invention has the advantage that it is not necessary to make an emulsion of the soft binder component. Surprisingly, it has now been found that open-graded asphalt compositions can be produced having a good resistance against fretting and in addition a good creep (rutting) and fatigue performance, by adding a hard binder component to a mixture of a non-emulsified soft binder component and aggregate at a relatively low temperature.